Many attempts have been made to simulate natural daylight by artificial means. It has been claimed, with some justification, that natural daylight is the preferred lighted environment. Thus, for example, in form 00112 8809L 150M (1990), the Duro-Test Corporation (of 9 Law Drive, Fairfield, N.J.) states that a good simulation of natural daylight " . . . encourages people to perform as never before because it promotes good vision . . . People see better and work better . . . " Thus, in form 0090 (1988), the Duro-Test Corporation states that light which " . . . simulates natural daylight . . . " is " . . . the perfect interior lighted environment . . . "
The Duro-Test Corporation markets the "VITA-LITE" fluorescent tube, which is described in U.S. Pat. No. 3,670,193. However, notwithstanding the claims of Duro-Test Corporation, such fluorescent tube is not a very good approximation of daylight. The light spectra obtainable from this fluorescent tube contains many high-energy, narrow-wavelength energy "spikes" with widths of less than 10 nanometers in the visible spectrum which do not appear in the spectrum of daylight and which adversely affect correct color perception by human beings. It appears that the spikes in the spectrum obtainable with this fluorescent tube within the visible spectra have a relative energy at least about 800 percent as great as the mean output of the lamp. By comparison, with natural daylight, the "spikes" or undulations in the spectrum are no greater than about 10 percent of the mean relative energy of the spectra.
Many other people have attempted to artificially simulate the spectrum of daylight, to no avail. Thus, for example, Westgate Enterprises (of 11988 Wilshire Blvd., Suite 104, Los Angeles, Calif.) markets a lamp called "CHROMALUX." Although this lamp produces a spectrum which does not contain as many high-energy "spikes" as the "VITA-LITE" lamp, it also does not produce a full spectrum; because it uses a neodymium dopant in the light envelope, the yellow portion of the spectrum (and other portions of the spectrum) is absent. Thus, in a 1990 brochure distributed by Westgate Enterprises, it is stated that "CHROMALUX is made of hand-blown glass containing neodymium . . . Neodymium is able to absorb yellow and other dulling portions of the spectrum."
In order to simulate daylight's spectrum, one must provide a full, even, and accurate distribution of light across the visible spectrum. The prior art discloses that this task is difficult, if not impossible. Thus, in Gunter Wyszecki's "Color Science: Concepts and Methods, Quantitative Data and Formulae," Second Edition (John Wiley & Sons, New York, 1982), it is stated (at pages 147-148) that " . . . the CIE has made no recommendations of artificial sources to realize any of the CIE illuminants D. The difficulty lies in the unique and rather jagged spectral distribution of daylight . . . No artificial sources with such spectral distribution are known, and modifying the spectral distributions of existing sources by placing filters in front of them or using other means has only been partially successful . . . " Thus, e.g., in D. L. MacAdam's "Color Measurement: Theme and Variations" (Springer-Verlag, New York, 1981), the author refers to the CIE's D65 illuminant, which is the standard spectra for daylight; at page 30, he states that " . . . the disadvantage of D65 is that no source of such light, except daylight itself, is available. Several artificial sources have been developed, but none gives a very close approximation to the CIE D65 . . . "
It is desirable to be able to simulate other daylight spectra, besides the D65 spectra. Thus, as is well known to those skilled in the art, the spectra of daylight will vary depending upon the daylight upon atmospheric conditions and solar altitude; see, e.g., S. T. Henderson's "Daylight and Its Spectrum," Second Edition(John Wiley & Sons, New York, 1977), the disclosure of which is hereby incorporated by reference into this specification.
It is also desirable to be able to simulate blackbody radiation in order, e.g., to calibrate light detectors. As is known to those skilled in the art, a blackbody is an ideal energy radiator which, at any specified temperature, emits in each part of the electromagnetic spectrum the maximum energy obtainable per unit time form any radiator due to its temperature alone and which also absorbs all of the energy which falls upon it. See, for example, the McGraw-Hill Encyclopedia of Science and Technology (McGraw-Hill Book Company, New York, 1977), particularly Volumes 2 (page 278), 6 (pages 419-423), and 7 (pages 55-56).
It is an object of this invention to provide an apparatus which is capable of producing a spectra simulating various daylights which spectra is substantially even and does not contain high-energy "spikes".
It is a further object of this invention to provide an apparatus which is capable of producing a full spectra which accurately simulates various daylights and which does not omit substantial portions of the visible spectrum.
It is a further object of this invention to provide an apparatus which is capable of simulating the spectra of other electromagnetic radiation, such as blackbody radiation, incandescent lights, monochromatic light, polychromatic light, and the like.